REFRIGERATORS
Of late years the frozen-meat trade has increased by leaps and bounds. Australia, New Zealand, Argentina, Canada, and the United States send millions of pounds' worth of mutton and beef across the water every year to help feed the populations of England and Europe.
In past times the live animals were sent, to be either killed when disembarked or fatted up for the market. This practice was expensive, and attended by much suffering of the unfortunate creatures if bad weather knocked the vessel about.
Refrigerating machinery has altered the traffic most fundamentally. Not only can more meat be sent at lower rates, but the variety is increased; and many other substances than flesh are often found in the cold stores of a ship—butter and fruit being important items.
Certain steamship lines, such as the Shaw, Savill, and Albion—plying between England and Australasia—include vessels specially built for the transport of vast numbers of carcases. Upwards of a million carcases have been packed into the hull of a single ship and kept perfectly fresh during the long six weeks' voyage across the Equator.
Every passenger-carrying steamer is provided with refrigerating rooms for the storage of perishable provisions; and as the comfort of the passengers, not to say their luxury, is bound up with these compartments, it will be interesting to glance at the method employed for creating local frost amid surrounding heat.
The big principle underlying the refrigerator is this—that a liquid when turned into gas absorbs heat (thus, to convert water into steam you must feed it with heat from a fire), and that as soon as the gas loses a certain amount of its heat it reverts to liquid form.
Now take ammonia gas. The "spirits of hartshorn" we buy at the chemist's is water impregnated with this gas. At ordinary living temperatures the water gives out the gas, as a sniff at the bottle proves in a most effective manner.
If this gas were cooled to 37·3° below zero it would assume a liquid state, i.e. that temperature marks its boiling point. Similarly steam, cooled to 212° Fahr., becomes water. Boiling point, therefore, merely means the temperature at which the change occurs.
Ammonia liquid, when gasifying, absorbs a great amount of heat from its surroundings—air, water, or whatever they may be. So that if we put a tumbler full of the liquid into a basin of water it would rob the water of enough heat to cause the formation of ice.
The refrigerating machine, generally employed on ships, is one which constantly turns the ammonia liquid into gas, and the gas back into liquid. The first process produces the cold used in the freezing-rooms. The apparatus consists of three main parts:—
(1) The compressor, for squeezing ammonia gas.
(2) The condenser, for liquefying the gas.
(3) The evaporator, for gasifying the liquid.
The compressor is a pump. The condenser, a tube or series of tubes outside which cold water is circulated. The evaporator, a spiral tube or tubes passing through a vessel full of brine. Between the condenser and evaporator is a valve, which allows the liquid to pass from the one to the other in proper quantities.
We can now watch the cycle of operations. The compressor sucks in a charge of very cold gas from the evaporator, and squeezes it into a fraction of its original volume, thereby heating it. The heated gas now passes into the condenser coils and, as it expands, encounters the chilling effects of the water circulating outside, which robs it of heat and causes it to liquefy.
It is next slowly admitted through the expansion valve into the evaporator. Here it gradually picks up the heat necessary for its gaseous form: taking it from the brine outside the coils, which has a very low freezing-point. The brine is circulated by pumps through pipes lining the walls of the freezing-room, and robs the air there of its heat until a temperature somewhat below the freezing-point of water is reached.
The room is well protected by layers of charcoal or silicate cotton, which are very bad conductors of heat. How the chamber strikes a novice can be gathered from the following description of a Cunard liner's refrigerating room. "It is a curious and interesting sight. It may be a hot day on deck, nearing New York, and everyone is going about in sun hats and light clothes. We descend a couple of flights of stairs, turn a key, and here is winter, sparkling in glassy frost upon the pale carcases of fowls and game, and ruddy joints of meat, crystallising the yellow apples and black grapes to the likeness of sweetmeats in a grocer's shop, gathering on the wall-pipes in scintillating coats of snow nearly an inch deep. You can make a snowball down here, if you like, and carry it up on deck to astonish the languid loungers sheltering from the sun under the protection of the promenade-deck roof. Such is the modern substitute for the old-time salt-beef cask and bags of dried pease!"
The larder is so near the kitchen that while below decks we may just peep into the kitchens, where a white-capped chef presides over an army of assistants. Inside a huge oven are dozens of joints turning round and round by the agency of an invisible electric-motor. But what most tickles the imagination is an electrical egg-boiling apparatus, which ensures the correct amount of cooking to any egg. A row of metal dippers, with perforated bottoms, is suspended over a trough of boiling water. Each dipper is marked for a certain time—one minute, two, three, four, and so on. The dippers, filled with eggs, are pushed down into the water. No need to worry lest they should be "done to a bullet," for at the expiry of a minute up springs the one-minute dipper; and after each succeeding minute the others follow in due rotation. Where 2,000 eggs or more are devoured daily this ingenious automatic device plays no mean part.